1. Field of the Invention
The invention relates to a self-exciting flyback converter that stabilizes the output voltage thereof by feeding back the output voltage through a photocoupler. The invention is directed, in more particular, to a self-exciting flyback converter that stops the switching operation when the output voltage thereof is decreased by an overload.
2. Description of the Related Art
Switching regulators that supply a stable DC voltage using as the input thereto a DC voltage obtained by rectifying the commercial AC power supply come in two types: a separately excited forward converter and a self-exciting flyback converter. The separately excited forward converter stabilizes the output voltage by pulse width modulation (PWM) control, while the self-exciting flyback converter is a switching regulator based on RCC. As a protective circuit for preventing breakage caused by an overload, conventional techniques are available for the both types of converters. The conventional exemplary protective circuits will be described below.
In the separately excited forward converter, the output voltage of an error amplifier fluctuates when the output voltage drops due to some abnormality such as an overload or a short-circuit. Thus, an excess in the output voltage of the error amplifier from a predetermined range gives an indication that there is some abnormality. Once such an indication has been given, it is judged whether the abnormality continues to stay for a predetermined short period. That is, an instantaneous level change due to effects such as noise is distinguished from the occurrence of abnormality through this judgment. When abnormality continues to be present for a predetermined period of time, the converter turns off the switching element while judging that there is some abnormality such as an overload or a short-circuit, so that the supply of the DC voltage is stopped (Conventional Example 1).
On the other hand, in the self-exciting flyback converter, a system in which the maximum current flowing through the switching transistor is limited based on a voltage obtained by adding a negative voltage to a positive voltage, the negative voltage being generated in the base coil when the switching transistor is in the off state and the positive voltage indicating the emitter current of the switching transistor. That is, since the negative voltage decreases when some abnormality such as an overload or a short-circuit occurs, the total voltage increases. Therefore, to reduce the total voltage, the emitter current of the switching transistor is controlled so as to decrease, thereby causing the output voltage to decrease. As a result, the current to be outputted can be limited to a predetermined value.
This design has the shortcoming that the current to be limited changes as the input voltage changes. To overcome this shortcoming, a design in which the maximum current flowing through the switching transistor is limited by the voltage obtained by adding up three voltages has been proposed. The three voltages are: a positive voltage generated in the base coil when the switching transistor is in the on state; a negative voltage generated in the base coil when the switching transistor is in the off state; and a positive voltage indicating the emitter current of the switching transistor. This design is advantageous in that the output current at the time of abnormality can be limited to a substantially constant value by the effect of the positive voltage generated in the base coil when the switching transistor is in the on state even if the input voltage changes.
In addition, this design characterized as adding up the three voltages is also advantageous in that the output current to be limited can be folded back in accordance with the setting of a ratio of addition. The foldback design is advantageous in that the output current at the time of abnormality can be limited to a small value (Conventional Example 2).
However, even in the case of employing Conventional Example 2, the current limited by the foldback design causes abnormal heating at an element suffering from a short-circuit or the like. For this reason, the following problem is encountered if the self-exciting flyback converter is used for, e.g., a domestic television set.
That is, when some abnormality occurs in the television set, there is a case where nobody is present around the television set and where such an abnormal television set is left unattended for a long period of time. This means that abnormal heating of the element persists for a long time, thereby imposing the problem that the breakage of the element affects other parts that are normally functioning.
On the other hand, there is no likelihood that the element will be broken in the case of employing Conventional Example 1 since the switching operation is immediately stopped at the time of abnormality in this case. However, when a comparison is made between the separately excited forward converter and the self-exciting flyback converter, the separately excited forward converter has a more complicated circuit configuration than that of the self-exciting flyback converter even excluding the protective circuit, and this makes the separately excited forward converter expensive in addition to the aforementioned disadvantage of complicated structure.